[racket] match and debug in Advanced Student

From: Prabhakar Ragde (plragde at uwaterloo.ca)
Date: Tue Nov 8 23:17:32 EST 2011

I never use the Debug button, but I let my students start using Advanced 
Student today, and of course they tried it out.

The following program induces a strange error when the Debug button is 

(match 3
   [`(,x) 3]
   [x 3])

The error is:
internal error: skipped binding is used

after which the program hangs, though it can be stopped with the Stop 
button. --PR

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